Watsonville to zap carp from Pinto Lake

The city will use the technique to target carp, bottom-feeders that stir up phosphorus-laden sediments, contributing to the growth of a toxic algae known as cyanobacteria.

The goal is to take out the largest carp and to drop overall numbers by at least two-thirds.

"That should make a significant dent in the amount of phosphorous available to the cyanobacteria," said Robert Ketley, city water quality specialist. "The smaller carp will naturally be culled by the lake's bass, egrets, herons and cormorants."

Cyanbacteria produces a toxin called microcystin that can cause rashes, nausea, diarrhea, liver and kidney damage. It's also been linked to deaths of livestock, pets and sea otters.

According to a 2012 study, nitrogen and phosphorus, which provide the nutrients for the algae to grow, are found in sediments deposited at the bottom of the 8,000-year-old lake between 50 and 100 years ago. As the carp suck insect larvae from the mud, they release the nutrients into the water.

Last year, the city sponsored a fishing contest called "Carpageddon" in an attempt to reduce the population. Ketley said about 1,000 carp, ranging from 2 pounds to 30 pounds, were pulled from the lake during the program, but that barely dented a population thought to number in the thousands.

The Coastal Conservancy, concerned about the link between toxins produced at Pinto and sea otter deaths, provided a $12,000 grant for the electrofishing.

The technique uses an electric field, which stuns the fish. The fish float to the surface, where they can be scooped into nets. Ketley said the carp will be put in an ethanol solution to kill them "quickly and humanely." Afterward, their bodies will be carted to the landfill.

Other fish will not be harmed, and will be able to swim away once the field is removed.

"E-fishing is the only approach that provides the kind of reductions we need without the risk of bycatch or avian mortality," Ketley said. "Biologists use the technique for sampling juvenile fish, including endangered species. Done right, it's the safest technique for selectively managing fish."

Ketley said studies at other lakes show carp are responsible for as much as 30 percent of the cyanobacteria growth. The electrofishing project isn't a permanent or complete solution, but it should help over the next decade.

"Eventually, the carp will re-establish themselves and we'll have to go back and zap them," Ketley said. "If all goes well that won't be necessary until at least 2024."